Organic electroluminescent device

ABSTRACT

An organic EL device  100  including a plurality of emitting layers ( 15 ) and ( 17 ) between a cathode ( 18 ) and ( 19 ) and an anode ( 12 ), each of the emitting layers ( 15 ) and ( 17 ) made of a host material having a triplet energy gap of 2.52 eV or more and 3.7 eV or less, and a dopant having a light emitting property related to a triplet state, the dopant containing a metal complex with a heavy metal.

This is a continuation application of U.S. application Ser. No.10/588,786, filed Jan. 19, 2007, which is a 371 of PCT/JP05/001802 filedon Feb. 8, 2005.

TECHNICAL FIELD

The invention relates to an organic electroluminescent device(hereinafter abbreviated as “organic EL device”). More particularly, theinvention relates to a highly efficient organic EL device.

BACKGROUND ART

An organic EL device using an organic substance is a promisingsolid-state emitting type inexpensive and large full-color displaydevice, and has been extensively developed. An EL device generallyincludes an emitting layer and a pair of opposing electrodes holding theemitting layer therebetween.

In the EL device, electrons and holes are injected into the emittinglayer from a cathode and an anode respectively upon application of anelectric field between the electrodes. The electrons and the holesrecombine in the emitting layer to produce an excited state, and theenergy is emitted as light when the excited state returns to the groundstate whereby the EL device emits light.

Various configurations have been known as the configuration of theorganic EL device. For example, use of an aromatic tertiary amine as amaterial for a hole-transporting layer has been disclosed for an organicEL device having the device configuration of “indium tin oxide(ITO)/hole-transporting layer/emitting layer/cathode” (seeJP-A-63-295695). This device configuration achieves a high luminance ofseveral hundreds cd/m² at an applied voltage of 20 V or less.

It has been reported that an emission efficiency of about 40 lm/W ormore is achieved at a luminance equal to or less than several hundredscd/m² by using an iridium complex (phosphorescent dopant) as a dopantfor an emitting layer (see Tsutsui et al., “Japanese Journal ofPhysics”, Vol. 38 (1999), p. 1502-1504).

However, since most phosphorescent organic EL devices emit green light,a phosphorescent organic EL device which emits blue light in variouscolors has been demanded. Moreover, an increase in the efficiency of thephosphorescent organic EL device has also been demanded.

When applying the organic EL device to a flat panel display or the like,the organic EL device is required to exhibit improved emissionefficiency and reduced power consumption. However, the above-mentioneddevice configuration has a disadvantage in that the emission efficiencysignificantly decreases accompanying an increase in luminance.Therefore, it is difficult to reduce the power consumption of the flatpanel display.

The invention was achieved in view of the above-described situation. Anobject of the invention is to provide a phosphorescent organic EL devicewhich exhibits high current efficiency and high luminous efficiency.

DISCLOSURE OF THE INVENTION

According to the invention, the following organic EL device is provided.

1. An organic EL device containing a plurality of emitting layersbetween a cathode and an anode, each of the emitting layers made of ahost material having a triplet energy gap of 2.52 eV or more and 3.7 eVor less, and a dopant having a light emitting property related to atriplet state, the dopant containing a metal complex with a heavy metal.2. The organic EL device according to 1, wherein the host materials ofthe emitting layers differ from each other.3. The organic EL device according to 1 or 2, wherein the host materialof at least one of the emitting layers is an organic compound containinga carbazolyl group.4. The organic electroluminescent device according to any one of 1 to 3,wherein the host material of at least one of the emitting layers is anorganic compound containing a carbazolyl group and a trivalent heteroring containing nitrogen.5. The organic EL device according to any one of 1 to 4, wherein thehost materials forming the emitting layers differ from each other inionization potential or electron affinity.6. The organic EL device according to any one of 1 to 5, wherein thehost materials of the emitting layers differ from each other inionization potential or electron affinity by 0.2 eV or more.7. The organic EL device according to any one of 1 to 6, wherein theemitting layers are stacked in contact with each other.8. The organic EL device according to any one of 1 to 7, wherein theoptical energy gap of the host material forming an emitting layer isequal to or smaller than the optical energy gap of the host materialforming the emitting layer adjacent in a direction of the anode.9. The organic EL device according to any one of 1 to 8, wherein anemitting layer containing a host material with a superiorhole-transporting property and an emitting layer containing a hostmaterial with a superior electron-transporting property are stacked.10. The organic EL device according to any one of 1 to 9, wherein atleast one of the emitting layers comprises a plural types of the dopantshaving a light emitting property related to a triplet state.11. The organic EL device according to any one of 1 to 10, wherein theemitting layer nearest to the cathode contains a first dopant that isdifferent from the dopant having a light emitting property related to atriplet state.12. The organic EL device according to 11, wherein the first dopant is ametal complex.13. The organic EL device according to 11 or 12, wherein when the devicecomprises an electron-transporting layer, the electron affinity of thefirst dopant is in a range between the electron affinity of anelectron-transporting material forming the electron-transporting layerand the electron affinity of the host material of the emitting layernearest to the cathode, and when the device does not comprise anelectron-transporting layer, the electron affinity of the first dopantis in a range between the work function of a cathode material and theelectron affinity of the host material of the emitting layer nearest tothe cathode.

The invention provides, a phosphorescent organic EL device, particularlyemitting light in a blue region, which exhibits high current efficiencyand high luminous efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an organic EL device according to Example 1.

FIG. 2 is a view showing an organic EL device according to Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

An organic EL device according to the invention includes a plurality ofemitting layers between a cathode and an anode. It is preferable that ahost material of each emitting layer differ from each other. The numberof interfaces between emitting layers is increased by providing aplurality of emitting layers. Since charges are accumulated around theinterfaces, recombination probability can be increased. Moreover, sincethe area in which the luminescent dopant described later exists isincreased, the emission region is increased. As a result, currentefficiency can be increased.

In the organic EL device according to the invention, the host materialforming the emitting layer has a triplet energy gap (Eg^(T)) of 2.52 eVor more and 3.7 eV or less, preferably 2.75 eV or more and 3.7 eV orless, still more preferably 2.80 eV or more and 3.7 eV or less, and evenmore preferably 2.90 eV or more and 3.7 eV or less. A host materialhaving a triplet energy gap within the above range allows the device toefficiently emit light irrespective of the color (blue to red) of aluminescent dopant.

In the organic EL device according to the invention, each emitting layerincludes a luminescent dopant having a light emitting property relatedto a triplet state and including a metal complex containing a heavymetal.

The presence of such a luminescent dopant allows emission from thetriplet state to contribute to EL emission, whereby the currentefficiency is increased.

In the organic EL device according to the invention, the emitting layersmay be adjacently stacked. The organic EL device according to theinvention may include an intermediate layer (e.g. charge adjustmentlayer) between the emitting layers. The material for the intermediatelayer is not particularly limited insofar as the material exhibits acharge transporting property. An inorganic conductive oxide layer or anorganic material known as a charge transporting material or an emittingmaterial may be used. The term “charge transporting property” means thata signal caused by each charge can be measured by a hole or electronmobility measurement method described later. A host material, holetransporting material, and electron transporting material describedlater may also be used. It is preferable that the intermediate layerhave a thickness equal to or less than the thickness of the emittinglayer.

It is preferable that the host material of each emitting layers differsfrom each other. It is more preferable that the host material of anemitting layer relatively near the anode be an organic compoundcontaining at least one carbazolyl group. It is still more preferablethat the host material of an emitting layer closer to the cathode thanthe emitting layer including the host material which is an organiccompound containing at least one carbazolyl group be an organic compoundcontaining a carbazolyl group and a trivalent hetero ring containingnitrogen.

It is preferable that the difference in ionization potential (Ip) orelectron affinity (Af) between the host materials of the emitting layersbe 0.2 eV or more, and still more preferably 0.3 eV or more.

This improves accumulation of charges, whereby high current efficiencyor luminous efficiency can be realized.

In the organic EL device according to the invention, it is preferablethat an emitting layer formed of a host material with a superior holetransporting property and an emitting layer formed of a host materialwith a superior electron transporting property be stacked. It is stillmore preferable that the emitting layers formed of such host materialsbe alternately stacked.

This improves accumulation of charges, whereby high current efficiencyor luminous efficiency can be realized.

In the invention, the term “superior hole transporting property” meansthat “hole mobility is greater than electron mobility”, and the term“superior electron transporting property” means that “electron mobilityis greater than hole mobility”.

The hole or electron mobility measurement method is not particularlylimited. As specific examples of the hole or electron mobilitymeasurement method, a time of flight method (method which calculates themobility from measured charge transit time in an organic film), a methodin which the mobility is calculated from the voltage characteristics ofthe space limited current, and the like can be given. In the time offlight method, light having a wavelength absorbed by the organic layermentioned below is applied to a structure including “electrode/organiclayer (layer formed of organic material forming electron transportinglayer or hole transporting layer)/electrode” to measure the transientcurrent time properties (transit time), and the electron or holemobility is calculated using the following expression.

Mobility=(thickness of organic film)²/(transit time−applied voltage)

Field intensity=(voltage applied to device)/(thickness of organic layer)

Note that methods disclosed in Electronic Process in Organic Crystals(M. Pope, C. E. Swenberg), Organic Molecular Solids (W. Jones), and thelike may also be used.

It is preferable that the host materials of the individual emittinglayers differ in ionization potential (Ip) or electron affinity (Af).

This improves accumulation of charges, whereby high current efficiencyor luminous efficiency can be realized.

It is preferable that the host material forming an emitting layer havean optical energy gap (Eg) equal to or smaller than the optical energygap (Eg) of the host material forming the emitting layer adjacent in thedirection of the anode. That is, it is preferable that emitting layerswhose number is “N” satisfy the following relationship.

Eg(N)≦Eg(N−1)≦ . . . ≦Eg(2)≦Eg(1)  (I)

Eg (x): optical energy gap of x-th (x is an integer of 1 or more and Nor less) emitting layer from the anode side

It is preferable that the host material forming an emitting layer have atriplet energy gap (Eg^(T)) equal to or smaller than the triplet energygap (Eg^(T)) of the host material forming the emitting layer adjacent inthe direction of the anode. That is, it is preferable that emittinglayers whose number is “N” satisfy the following relationship.

Eg^(T)(N)≦Eg ^(T)(N−1)≦ . . . ≦Eg^(T)(2)≦Eg^(T)(1)  (II)

Eg^(T) (x): triplet energy gap of x-th (x is an integer of 1 or more andN or less) emitting layer from the anode side If the relationship shownby the expression (I) or (II) is satisfied, the recombination energy canbe efficiently stored in the emitting layers to contribute to emission,whereby a device exhibiting a high current efficiency can be realized.

In the organic EL device according to the invention, the host materialsand the luminescent dopants forming the emitting layers are notparticularly limited insofar as the above conditions are satisfied.

As the host material, a compound having a carbazolyl group ispreferable. It is preferable to form a stacked structure or a multilayerstructure of a hydrocarbon derivative of an organic compound having acarbazolyl group, and an electron-attracting substituent derivative ofan organic compound having a carbazolyl group or a nitrogen-containingderivative of an organic compound having a carbazolyl group. Afluorine-containing derivative may be used instead of thenitrogen-containing derivative.

As specific examples of the compound having a carbazolyl group,compounds disclosed in JP-A-10-237438, Japanese Patent Application No.2003-042625, Japanese Patent Application No. 2002-071398, JapanesePatent Application No. 2002-081234, Japanese Patent Application No.2002-299814, Japanese Patent Application No. 2002-360134, and the likecan be given. Specific compounds are given below.

A compound having a carbazolyl group (described later) which may be usedas the electron transporting material may also be used as the hostmaterial.

As examples of the host material exhibiting an excellent holetransporting property, compounds disclosed in JP-A-10-237438 andJapanese Patent Application No. 2003-042625 can be given. As examples ofthe host material exhibiting an excellent electron transportingproperty, compounds disclosed 2002-299814, and Japanese PatentApplication No. 2002-360134 can be given.

The host material may be a compound given below.

It is preferable that the luminescent dopant function as a dopant whichemits light due to the triplet state at room temperature. As preferableexamples of the heavy metal contained in the luminescent dopant, Ir, Pt,Pd, Ru, Rh, Mo, and Re can be given. As examples of the ligandcoordinated to the heavy metal, a ligand which is coordinated or bondedto a metal at C or N(CN ligand) and the like can be given. As specificexamples of such a ligand, the following compounds and substitutedderivatives thereof can be given.

As examples of the substituent for the substituted derivatives, an alkylgroup, alkoxy group, phenyl group, polyphenyl group, naphthyl group,fluoro (F) group, trifluoromethyl (CF₃) group, and the like can begiven.

As examples of a blue emitting ligand, the following compounds and thelike can be given.

In the organic EL device according to the invention, it is preferablethat at least one of the emitting layers include a plurality ofluminescent dopants in order to realize a device exhibiting a highcurrent efficiency.

It is preferable that the emitting layer nearest to the cathode includea first dopant different from the luminescent dopant. The first dopantneed not emit light and is not particularly limited insofar as the firstdopant is an organic compound which improves injection of electrons intothe emitting layer. As the first dopant, an organic compound having anelectron-attracting substituent (e.g. cyano group (CN), nitro group(NO₂), or quinolyl group) is preferable.

As specific examples of such an organic compound, nitrogen-containingorganic compounds (e.g. oxazole derivatives) and fluorine-substitutedcompounds thereof, compounds having a Cz-hetero ring (Cz: carbazolylgroup) disclosed in Japanese Patent Application No. 2002-071398,Japanese Patent Application No. 2002-081234, Japanese Patent ApplicationNo. 2002-299814, and Japanese Patent Application No. 2002-360134,hydrocarbon organic compounds (e.g. alkyl-substituted compound of styrylderivative), electron-attracting group substituted hydrocarbons (e.g.cyano group, fluoro group, pyridyl group, pyrazinyl group, pyrimidinylgroup, or pyridanyl group derivative of styryl derivative), metalcomplexes, and the like can be given. Of these, the metal complexes arepreferable.

As specific examples of preferable metal complexes, organic compoundsdisclosed in JP-A-5-258860 and compounds shown by the following formula(1) can be given.

wherein R¹ represents an alkyl group, oxy group, or amino group, R² andR³ individually represent a hydrogen atom, an alkyl group, oxy group, oramino group, R⁴, R⁵, and R⁶ individually represent a hydrogen atom, analkyl group, oxy group, amino group, cyano group, halogen group,a-haloalkyl group, α-haloalkoxy group, amide group, or sulfonyl group,and L represents a group shown by the following formula (2) or (3).

wherein R⁷ to R²⁶ individually represent a hydrogen atom or ahydrocarbon group.

Specific examples of the metal complex shown by the formula (1) aregiven below.

When the device includes an electron transporting layer, it ispreferable that the electron affinity of the first dopant be in a rangebetween the electron affinity of the electron transporting materialforming the electron transporting layer and the electron affinity of thehost material of the emitting layer nearest to the cathode. When thedevice does not include an electron transporting layer, it is preferablethat the electron affinity of the first dopant be in a range between thework function of the cathode material and the electron affinity of thehost material of the emitting layer nearest to the cathode. Thisconfiguration allows injection of electrons into the emitting layer tobe improved, whereby the emission efficiency can be increased.

As examples of the electron transporting material, the metal complexesshown by the above formula (1), organic compounds disclosed in JapanesePatent Application No. 2002-071398, Japanese Patent Application No.2002-081234, Japanese Patent Application No. 2002-299814, and JapanesePatent Application No. 2002-360134, and the like can be given.

A compound having a carbazolyl group may also be used as the electrontransporting material. Specific examples of such a compound are givenbelow.

As examples of the configuration of the organic EL device according tothe invention, the following configurations (a) to (g) can be given.

(a) Anode/multi-layered emitting layer/electron-transportinglayer/cathode(b) Anode/hole-transporting layer/multi-layered emittinglayer/electron-transporting layer/cathode(c) Anode/hole-injecting layer/hole-transporting layer/multi-layeredemitting layer/electron-transporting layer/cathode(d) Anode/emitting layer/organic layer/emittinglayer/electron-transporting layer/cathode(e) Anode/multi-layered emitting layer/organic layer/multi-layeredemitting layer/electron-transporting layer/cathode(f) Anode/hole-transporting layer/multi-layered emitting layer/organiclayer/multi-layered emitting layer/electron-transporting layer/cathode(g) Anode/hole-injecting layer/hole-transporting layer/multi-layeredemitting layer/organic layer/multi-layered emittinglayer/electron-transporting layer/cathode

The emitting layer in the organic EL device according to the inventionis defined as an organic layer containing the above luminescent dopant.The concentration of the luminescent dopant added is not particularlylimited. It is preferably 0.1 to 30 wt %, and more preferably 0.1 to 10wt %.

The organic EL device according to the invention is preferably supportedby a substrate. The layers may be stacked on the substrate in the orderfrom the anode to the cathode, or may be stacked on the substrate in theorder from the cathode to the anode.

It is preferable that at least one of the anode and the cathode beformed of a transparent or translucent substance in order to efficientlyout-couple light from the emitting layer.

The material for the substrate used in the invention is not particularlylimited. A known material used for an organic EL device such as glasses,transparent plastics, or quartz may be used.

As the material for the anode used in the invention, a metal, alloy, orelectric conductive compound having a work function as large as 4 eV ormore, or a mixture of these materials is preferably used. As specificexamples of such a material, metals such as Au and dielectrictransparent materials such as CuI, ITO, SnO₂, and ZnO can be given.

The anode may be formed by forming a thin film of the above-mentionedmaterial by deposition, sputtering method, or the like.

When out-coupling light from the emitting layer through the anode, it ispreferable that the anode have a transparency of more than 10%.

The sheet resistance of the anode is preferably several hundredsohm/square or less.

The thickness of the anode is usually 10 nm to 1 micron, and preferably10 to 200 nm, although the thickness varies depending on the material.

As the material for the cathode used in the invention, a metal, alloy,or electric conductive compound having a work function as small as 4 eVor less, or a mixture of these materials is preferably used. As specificexamples of such a material, sodium, lithium, aluminum, magnesium/silvermixture, magnesium/copper mixture, Al/Al₂O₃, indium, and the like can begiven.

The cathode may be formed by forming a thin film of the above-mentionedmaterial by deposition, sputtering, or the like.

When outcoupling light from the emitting layer through the cathode, itis preferable that the cathode have a transparency of more than 10%.

The sheet resistance of the cathode is preferably several hundredsohm/square or less.

The thickness of the cathode is usually 10 nm to 1 micron, andpreferably 50 to 200 nm, although the thickness varies depending on thematerial.

In the organic EL device according to the invention, a hole-injectinglayer, a hole-transporting layer, an electron-injecting layer, and thelike may be provided, as required, in order to further increase current(or luminous) efficiency. The materials for these layers are notparticularly limited. A known organic material for an organic EL may beused. As specific examples of such a material, amine derivatives,stilbene derivatives, silazane derivatives, polysilane, anilinecopolymers, and the like can be given.

Hole-transporting materials include compounds mentioned in JapanesePatent Application Nos. 2002-071397, 2002-080817, 2002-083866,2002-087560, 2002-305375 and 2002-360134.

In the invention, it is preferable to add an inorganic material to thehole-injecting layer, the hole-transporting layer, theelectron-injecting layer or the electron-transporting layer. As examplesof the inorganic material, metal oxides and the like can be given.

An inorganic material may be preferably used for the hole-injectinglayer or the hole-transporting layer.

An inorganic material may be used between the electron-transportinglayer and the metal cathode in order to increase current (or luminous)efficiency. As specific examples of the inorganic material, fluoridesand oxides of alkali metals such as Li, Mg, and Cs can be given.

The method of fabricating the organic EL device according to theinvention is not particularly limited. The organic EL device accordingto the invention may be fabricated using a known fabrication method usedfor an organic EL device. In more detail, each layer may be formed byvacuum deposition, casting, coating, spin coating, or the like. Eachlayer may also be formed by casting, coating, or spin coating using asolution prepared by dispersing an organic material for each layer in atransparent polymer such as polycarbonate, polyurethane, polystyrene,polyallylate, or polyester, as well as simultaneous deposition of anorganic material and a transparent polymer.

EXAMPLES

The invention is described below in more detail by way of examples. Notethat the invention is not limited to the following examples.

Compounds used in the examples were produced by the methods disclosed inJP-A-10-237438, Japanese Patent Application Nos. 2003-042625,2002-071398, 2002-081234, 2002-299814, 2002-360134, 2002-071397,2002-080817, 2002-083866, 2002-087560, and 2002-305375.

The parameters shown in the table were measured by the followingmethods.

(1) Ionization Potential (Ip)

Light (excitation light) from a deuterium lamp dispersed by amonochromator was irradiated to a material, and the resultingphotoelectric emission was measured using an electrometer. Theionization potential was determined by calculating the photoelectricemission threshold value from the photoelectric emission photon energycurve obtained using an extrapolation method. As the measuringinstrument, an atmosphere ultraviolet photoelectron spectrometer “AC-1”(manufactured by Riken Keiki Co., Ltd.) was used.

(2) Optical Energy Gap (Eg)

Light of which the wavelength was resolved was irradiated to a toluenediluted solution of each material, and the optical energy gap wasdetermined by conversion from the maximum wavelength of the absorptionspectrum. As the measuring instrument, a spectrophotometer (“U-3400”manufactured by Hitachi, Ltd.) was used.

(3) Triplet Energy Gap (Eg^(T))

The triplet energy gap (Eg^(T) (Dopant)) was determined by the followingmethod. An organic material was measured by a known phosphorescencemeasurement method (e.g. method described in “The World ofPhotochemistry” (edited by The Chemical Society of Japan, 1993), page50). In more detail, an organic material was dissolved in a solvent(sample 10 micromol/l, EPA (diethyl ether:isopentane:ethanol=5:5:2(volume ratio), each solvent was spectrum grade) to obtain aphosphorescence measurement sample. After cooling the sample placed in aquartz cell to 77K, excitation light was irradiated to the sample, andthe resulting phosphorescence was measured with respect to thewavelength. A tangent was drawn to the rise of the phosphorescencespectrum on the shorter wavelength side, and the value obtained byconverting the wavelength into the energy value was taken as the tripletenergy gap (Eg^(T)). The triplet energy gap was measured using a“F-4500” fluorescence spectrophotometer (manufactured by Hitachi, Ltd.)and optional low temperature measurement equipment. Note that themeasuring instrument is not limited thereto. The triplet energy gap maybe measured by combining a cooling device, a low temperature container,an excitation light source, and a light receiving device.

In the examples, the wavelength was converted using the followingexpression.

Eg^(T)(eV)=1239.85/λ_(edge)

The meaning of “λ_(edge)” is as follows. When the phosphorescencespectrum is expressed in which the vertical axis indicates thephosphorescence intensity and the horizontal axis indicates thewavelength, and a tangent is drawn to the rise of the phosphorescencespectrum on the shorter wavelength side, “λ_(edge)” is the wavelength atthe intersection of the tangent and the horizontal axis. The unit for“λ_(edge)” is nm.

(4) Electron Affinity (Af)

The electron affinity was calculated from the expression Af=Ip−Eg, usingthe measured values, Ip and Eg.

Example 1

An organic EL device shown in FIG. 1 was fabricated as follows.

A glass substrate 11, measuring 25 mm×75 mm×1.1 mm thick, with an ITOtransparent electrode (anode) 12 (manufactured by Geomatics Co.) wassubjected to ultrasonic cleaning in isopropyl alcohol for 5 minutes andthen to UV ozone cleaning for 30 minutes. The cleaned glass substrate 11with transparent electrode lines was mounted on a substrate holder in avacuum deposition device. First, a 100 nm thick film ofN,N′-bis(N,N′-diphenyl-4-aminophenyl)-N,N-diphenyl-4,4′-diamino-1,1′-biphenyl(hereinafter referred to as TPD 232 film) 13 was formed by resistanceheating deposition on the surface where the transparent electrode lineswere formed, so as to cover the transparent electrode 12. This TPD 232film 13 functioned as a hole-injecting layer (a hole-transportinglayer).

After the formation of the TPD 232 film 13, a 10 nm thickhole-transporting layer (hereinafter referred to as HTM) 14 was formedby resistance heating deposition. After the formation of thehole-transporting layer 14, a host material 1 (Host No. 1, Eg=3.53 eV,Eg^(T)=2.86 eV, Ip=5.59 eV, Af=2.06 eV) and a luminescent dopant(FIrpic, Eg=2.8 eV, Eg^(T)=2.7 eV, Ip=5.6 eV, Af=2.8 eV, shown below)were co-deposited by resistance heating to form a 20 nm thick film 15thereon. The concentration of FIrpic was 7.5 wt %. This Host No. 1:FIrpic film 15 functioned as an emitting layer.

A 1 nm thick layer 16 made of the host material 1 was formed on thisfilm. The film 16 functioned as a charge adjustment layer, whereby anelectric charge can be accumulated well in the emitting layer and acurrent efficiency of the device increases.

Moreover, a host material 2 (Host No. 2, Eg=3.55 eV, Eg^(T)=2.90 eV,Ip=5.71 eV, Af=2.16 eV) and FIrpic were co-deposited by resistanceheating to form a 20 nm thick film 17 thereon. The concentration ofFIrpic was 7.5 wt %. This Host No. 2: FIrpic film 17 functioned as anemitting layer.

Thereafter, a 0.1 nm thick electron-injecting electrode (cathode) 18 ofLiF was formed at a film-formation rate of 1 Å/minute. A metal Al (workfunction: 4.2 eV) was deposited on the LiF layer 18 to form a 130 nmthick metal cathode 19, thereby fabricating an organic EL device 100.

Example 2

An organic EL device was fabricated in the same manner as in Example 1except that the following PC-8 was introduced by resistance heatingdeposition to a thickness of 30 nm as an electron-transporting layer onan emitting layer made of the host material 2: FIrpic.

Example 3

An organic EL device shown in FIG. 2 was fabricated as follows.

A glass substrate 21, measuring 25 mm×75 mm×1.1 mm thick, with an ITOtransparent electrode (anode) 22 (manufactured by Geomatics Co.) wassubjected to ultrasonic cleaning in isopropyl alcohol for 5 minutes andthen to UV ozone cleaning for 30 minutes. The cleaned glass substrate 21with transparent electrode lines was mounted on a substrate holder in avacuum deposition device. First, a 100 nm thick TPD 232 film 23 wasformed by resistance heating deposition on the surface where thetransparent electrode lines were formed, so as to cover the transparentelectrode 22. The TPD 232 film 23 functioned as a hole-injecting(hole-transporting) layer.

After the formation of the TPD 232 film 23, a 10 nm thickhole-transporting layer (HTM) 24 was formed by resistance heatingdeposition. After the formation of the hole-transporting layer 24, ahost material 1 and FIrpic were co-deposited by resistance heating toform a 20 nm thick emitting layer 25 (emitting layer) thereon. Theconcentration of FIrpic was 7.5 wt %.

After the formation of the TPD 232 film 23, a host material 3 (Host No.3, Eg=3.55 eV, Eg^(T)=2.91 eV, Ip=5.40 eV, Af=1.85 eV) and FIrpic wereco-deposited by resistance heating to form a 20 nm thick film 26thereon. The concentration of FIrpic was 7.5 wt %. This Host No. 3:FIrpic film 26 functioned as an emitting layer.

Then, a 30 nm thick electron-transporting layer 27 (PC-8) was formed byresistance heating deposition on the emitting layer 26.

Thereafter, a 0.1 nm thick electron-injecting electrode (cathode) 28 ofLiF was formed at a film-formation rate of 1 Å/minute. A metal Al wasdeposited on the LiF layer 28 to form a 130 nm thick metal cathode 29,thereby fabricating an organic EL device 200.

Example 4

A glass substrate, measuring 25 mm×75 mm×1.1 mm thick, with an ITOtransparent electrode (manufactured by Geomatics Co.) was subjected toultrasonic cleaning in isopropyl alcohol for 5 minutes and then to UVozone cleaning for 30 minutes. The cleaned glass substrate withtransparent electrode lines was mounted on a substrate holder in avacuum deposition device. First, a 100 nm thick TPD 232 film was formedby resistance heating deposition on the surface where the transparentelectrode lines were formed, so as to cover the transparent electrode.The TPD 232 film functioned as a hole-injecting (hole-transporting)layer.

After the formation of the TPD 232 film, a 10 nm thick hole-transportinglayer (HTM) was formed by resistance heating deposition thereon.

After the formation of the hole-transporting layer, the host material 1and FIrpic were co-deposited by resistance heating to form a 20 nm thickfilm (emitting layer) thereon. The concentration of FIrpic was 7.5 wt %.

Moreover, a host material 4 (Host No. 4, Eg=3.16 eV, Eg^(T)=2.78 eV,Ip=5.84 eV, Af=2.66 eV) and FIrpic were co-deposited by resistanceheating to form a 20 nm thick film thereon. The concentration of FIrpicwas 7.5 wt %. This Host No. 4: FIrpic film functioned as an emittinglayer.

Then, a 30 nm thick electron-transporting layer (Alq, Af=3.0 eV, shownbelow) was formed by resistance heating deposition on the emittinglayer.

Thereafter, a 0.1 nm thick electron-injecting electrode (cathode) of LiFwas formed at a film-formation rate of 1 Å/minute. A metal Al wasdeposited on the LiF layer to form a 130 nm thick metal cathode, therebyfabricating an organic EL device.

Example 5

A device was fabricated by the same steps as in Example 4 except thatthe host material 4 was changed to a host material (Host No. 5, Eg=3.57eV, Eg^(T)=2.89 eV, Ip=5.60 eV, Af=2.03 eV).

Example 6

A device was fabricated by the same steps as in Example 4 except thatthe host material 4 was changed to a host material 6 (Host No. 6,Eg=3.56 eV, Eg^(T)=2.87 eV, Ip=5.85 eV, Af=2.29 eV).

Example 7

A device was fabricated by the same steps as in Example 3 except thatthe host material 1 was changed to the host material 3 and the hostmaterial 3 to the host material 4 respectively.

Example 8

A glass substrate, measuring 25 mm×75 mm×1.1 mm thick, with an ITOtransparent electrode (manufactured by Geomatics Co.) was subjected toultrasonic cleaning in isopropyl alcohol for 5 minutes and then to UVozone cleaning for 30 minutes. The cleaned glass substrate withtransparent electrode lines was mounted on a substrate holder in avacuum deposition device. First, a 100 nm thick TPD 232 film was formedby resistance heating deposition on the surface where the transparentelectrode lines were formed, so as to cover the transparent electrode.The TPD 232 film functioned as a hole-injecting (hole-transporting)layer.

After the formation of the TPD 232 film, a 10 nm thick hole-transportinglayer (HTM) was formed by resistance heating deposition.

After the formation of the hole-transporting layer, a host material 1and FIrpic were co-deposited by resistance heating to form a 30 nm thickfilm (emitting layer) thereon. The concentration of FIrpic was 7.5 wt %.

Moreover, the host material 1, FIrpic and PC-8 (Af=2.7 eV) wereco-deposited by resistance heating to form a 10 nm thick film (emittinglayer) thereon. The concentration of FIrpic and PC-8 each were 7.5 wt %.

Thereafter, a 0.1 nm thick electron-injecting electrode (cathode) of LiFwas formed at a film-formation rate of 1 Å/minute. A metal Al (workfunction, 4.2 eV) was deposited on the LiF layer to form a 130 nm thickmetal cathode, thereby fabricating an organic EL device.

Comparative Example 1

A glass substrate, measuring 25 mm×75 mm×1.1 mm thick, with an ITOtransparent electrode (manufactured by Geomatics Co.) was subjected toultrasonic cleaning in isopropyl alcohol for 5 minutes and then to UVozone cleaning for 30 minutes. The cleaned glass substrate withtransparent electrode lines was mounted on a substrate holder in avacuum deposition device. First, a 100 nm thick TPD 232 film was formedby resistance heating deposition on the surface where the transparentelectrode lines were formed, so as to cover the transparent electrode.The TPD 232 film functioned as a hole-injecting (hole-transporting)layer.

After the formation of the TPD 232 film, a 10 nm thick hole-transportinglayer (HTM) was formed by resistance heating deposition.

After the formation of the hole-transporting layer, the host material 1and FIrpic were co-deposited by resistance heating to form a 40 nm thickfilm thereon. The concentration of FIrpic was 7.5 wt %.

Moreover, an electron-transporting layer (Alq) with a certain thickness(30 nm) was formed by resistance heating deposition on the emittinglayer.

Thereafter, a 0.1 nm thick electron-injecting electrode (cathode) of LiFwas formed at a film-formation rate of 1 Å/minute. A metal Al wasdeposited on the LiF layer to form a 130 nm thick metal cathode, therebyfabricating an organic EL device.

(Evaluation of Organic EL Device)

A current density, luminance, efficiency and chromaticity of the organicEL devices obtained in the examples and the comparative example weremeasured by applying a certain DC voltage. A current efficiency (═O(luminance)/(current density)) at a luminance of about 100 cd/m² wascalculated. The results were shown in Table 1.

TABLE 1 Volt- Current Current Luminous age density efficiency efficiency(V) (mA/cm²) CIE- (x, y) (cd/A) (lm/W) Example 1 9.0 0.45 (0.180, 0.431)22.2 7.76 Example 2 8.0 0.4 (0.175, 0.431) 25 9.8 Example 3 7.5 0.4(0.175, 0.431) 25 10.4 Example 4 8.5 0.4 (0.175, 0.431) 25 9.2 Example 58.0 0.45 (0.175, 0.431) 22.2 8.73 Example 6 8.3 0.4 (0.175, 0.431) 259.46 Example 7 7.0 0.38 (0.175, 0.431) 26.3 11.8 Example 8 8.5 0.5(0.175, 0.431) 20 7.4 Comparative 8.0 1.01 (0.20, 0.41) about 10 3.9Example 1

The results reveal that the invention realized a device with a highercurrent efficiency than conventional devices which have the sameemission color.

INDUSTRIAL APPLICABILITY

The organic EL device of the invention can be used for an informationdisplay device, a display device for automobiles, a lighting and so onbecause its luminous efficiency is high at a high luminance and theelectric power consumption is low. Specifically, it can be suitably usedfor a flat luminescent body for wall hanging TVs, a back lighting sourcefor displays and so on.

The contents of the documents and publications cited in the descriptionare incorporated herein by reference.

1. An organic electroluminescent device comprising a plurality ofemitting layers and a charge adjustment layer between a cathode and ananode, each of the emitting layers comprising a host material having atriplet energy gap of 2.52 eV or more and 3.7 eV or less, and a dopanthaving a light emitting property related to a triplet state, the dopantcomprising a metal complex with a heavy metal, and the charge adjustmentlayer being between emitting layers.
 2. The organic electroluminescentdevice according to claim 1, wherein the material of the chargeadjustment layer is the same as any one of the host materials of theplurality of emitting layers.
 3. The organic electroluminescent deviceaccording to claim 1, wherein the host materials of the emitting layersdiffer from each other.
 4. The organic electroluminescent deviceaccording to claim 1, wherein the host material of at least one of theemitting layers is an organic compound containing a carbazolyl group. 5.The organic electroluminescent device according to claim 1, wherein thehost material of at least one of the emitting layers is an organiccompound containing a carbazolyl group and a trivalent hetero ringcontaining nitrogen.
 6. The organic electroluminescent device accordingto claim 1, wherein the host materials forming the emitting layersdiffer from each other in ionization potential or electron affinity. 7.The organic electroluminescent device according to claim 1, wherein thehost materials of the emitting layers differ from each other inionization potential or electron affinity by 0.2 eV or more.
 8. Theorganic electroluminescent device according to claim 1, wherein theoptical energy gap of the host material forming an emitting layer isequal to or smaller than the optical energy gap of the host materialforming the emitting layer adjacent in a direction of the anode.
 9. Theorganic electroluminescent device according to claim 1, wherein anemitting layer comprising a host material with a superiorhole-transporting property and an emitting layer comprising a hostmaterial with a superior electron-transporting property are stacked. 10.The organic electroluminescent device according to claim 1, wherein atleast one of the emitting layers comprises plural types of the dopantshaving a light emitting property related to a triplet state.
 11. Theorganic electroluminescent device according to claim 1, wherein theemitting layer nearest to the cathode comprises a first dopant that isdifferent from the dopant having a light emitting property related to atriplet state.
 12. The organic electroluminescent device according toclaim 11, wherein the first dopant is a metal complex.
 13. The organicelectroluminescent device according to claim 11, wherein when the devicecomprises an electron-transporting layer, the electron affinity of thefirst dopant is in a range between the electron affinity of anelectron-transporting material forming the electron-transporting layerand the electron affinity of the host material of the emitting layernearest to the cathode, and when the device does not comprise anelectron-transporting layer, the electron affinity of the first dopantis in a range between the work function of a cathode material and theelectron affinity of the host material of the emitting layer nearest tothe cathode.